1. Field of the Invention
The present invention relates to video switching equipment, and in particular, to a signal equalizer device, system, and method for shielded and unshielded twisted pair-based video switching.
2. Description of Related Art
A video switch allows a user to route a plurality of video source inputs to a single or plurality of video displays. Video switching is typically performed by centralized equipment having input and output ports through which video signals interface. However, there are a number of obstacles in displaying video with high quality. Video switching devices are subject to signal arrival skew, which occurs when multiple channels of video are propagated through physically separate transmission lines. Also, when a video source and display are connected through transmission lines, video signal quality suffers as the length of the transmission line increases. In a video switching environment, input signals from multiple sources that are otherwise identical will appear inconsistent if transported over cables of varying length. Furthermore, undesirable variations in video quality will appear as the video switch selects different inputs to be displayed. When the input signals are of varying quality, video switching performance is further degraded.
Transmission lines, such as coaxial cables, fiber optic cables, and unshielded twisted pair cables, impose losses that are manifested as attenuation and high frequency roll off, which are the combined result of ohmic, skin effect, dielectric and radiation losses. High-resolution analog signals that contain a very broad spectrum of energy, often occupying 20 octaves or more per channel, are especially susceptible to attenuation and roll off even in moderately long cables. Visually, cable losses are manifested as horizontal aperture distortion (smearing) and a loss of fine detail resolution.
Conventionally, the transportation of high-resolution video over long distances has required the use of fiber optic transmission or multiple coaxial cables. However, these cables are both costly and cumbersome, often requiring the installation of as many as five individual fibers or coaxial cables. Termination of coaxial cables is difficult and expensive in comparison to other cable types such as shielded and unshielded twisted pair (STP, UTP) cables.
Recently, UTP cable lines, such as category cables, have been employed in the transportation of high bandwidth applications, such as video. Category cables, such as CAT-5, CAT-5e and CAT-6, are cables that combine four separate, unshielded, twisted pair transmission lines into a single sheath. Category cables are typically used in telephony and local area network applications and are commonly referred to as Ethernet cables. UTP cables provide both the low cost and the convenience of a single, easily terminated cable.
However, video transmission using UTP cables presents problems that prevent widespread adoption over the more commonly used coaxial cables. Many UTP cables employ varying twist rates for each pair of conductors to improve crosstalk characteristics. Improved crosstalk comes at the expense of varying propagation speeds between individual pairs within a given cable sheath. If not corrected, video signals will display artifacts as horizontally displaced, colored fringing on vertical image elements that are visible in both graphics and text. UTP cables are also susceptible to signal arrival skew. While UTP cables are inexpensive and easier to install and maintain than coaxial cables, current solutions have not provided comparable performance to coaxial cables for high-resolution video over moderate to long distances.
In order to solve the problems present in video switching applications, “peaking” adjustments have been incorporated into video switches and video distribution amplifiers. These simplistic adjustments, typically accomplished using elementary single pole, shelved high pass networks, increase high frequency gain near the top of the expected signal bandwidth. In this manner, signals are equalized such that video signals transported over long cable runs are made to behave like video signals transported over short cable runs. Such compensation for cable length is referred to as positive equalization (positive EQ). However, the conventional solutions are effective only for low bandwidth, low-resolution, NTSC type video sources and switches. Only perfunctory compensation for moderately long source to input port cables are provided. Utilizing only a single time constant, the resulting transfer functions obtained conventionally do not provide the accuracy and frequency response necessary for high resolution analog video signals. The current solutions for providing positive EQ are inadequate for equalizing high-resolution video signals and video signals transported over long distances.